Io is Jupiter's fourth largest moon. With more than 400 active volcanoes, the satellite is the most geologically active object in the solar system.

In 2015, Europa passed in front of Io, casting a shadow on the Jovian moon. Without the interference of reflected light, researchers were able to more accurately measure ultraviolet radiation, which is thoroughly absorbed by Io's surface.

The data revealed a stark temperature gradient inside Loki Patera, suggesting lava inside the molten lake had overturned in the form of two waves moving west to east at a rate of 3,300 feet per day.

Periodic waves likely explain why astronomers have long observed a dimming and brightening of Io's main hot spot, which stretches 127 miles across.

"If Loki Patera is a sea of lava, it encompasses an area more than a million times that of a typical lava lake on Earth," lead study author Katherine de Kleer, grad student at the University of California, Berkeley, said in a news release. "In this scenario, portions of cool crust sink, exposing the incandescent magma underneath and causing a brightening in the infrared."

Researchers have previously mapped Loki Patera using ground-based observatories, but the occupation of Io by Europa allowed for much improved resolution during the most recent survey.

The volcanic lake is shaped like a donut, and the latest observations show two waves of overturning lava sweeping around either side of the donut hole. Interestingly, researchers believe the pair of waves began at two different times.

"The velocity of overturn is also different on the two sides of the island, which may have something to do with the composition of the magma or the amount of dissolved gas in bubbles in the magma," de Kleer said. "There must be differences in the magma supply to the two halves of the patera, and whatever is triggering the start of overturn manages to trigger both halves at nearly the same time but not exactly. These results give us a glimpse into the complex plumbing system under Loki Patera."